2 Molecular/Atomic InteractionsU(r)rDHcrecDHprepEactChemisorptionPhysisorptionIn most of the case: Physisorption before ChemisorptionChemisorptionFormation of moleculesShort DistancePhysisorptionNo molecules formationLong Distance

3 Types of interactions Physisorption ChemisorptionExothermiclDHp l < 20 kJ/mol> 1 layer adsorbedNot SpecificKinetic: Fast - since it is a non-activated processChemisorptionlDHc l > 100 kJ/molOnly 1 layer adsorbedSpecificKinetic: Depends of the activation energyChemisorption PhysisorptionTemperature Range(over which adsorption occurs) Virtually unlimited(but a given molecule may effectively adsorb only over a small range) Near or below the condensation point of the gas(e.g. Xe < 100 K, CO2 < 200 K)Adsorption Enthalpy Wide range (related to the chemical bond strength) - typically kJ mol-1 Related to factors like molecular mass and polarity but typically 5-40 kJ mol-1 (i.e. ~ heat of liquefaction)Crystallographic Specificity(variation between different surface planes of the same crystal) Marked variation between crystal planes Virtually independent of surface atomic geometryNature of Adsorption Often dissociativeMay be irreversible Non-dissociativeReversibleSaturation Uptake Limited to one monolayer Multilayer uptake possibleKinetics of Adsorption Very variable - often an activated process Fast - since it is a non-activated process

4 Covalent/Ionic Directional Partial Exchange of electronsCovalent bondingDirectionalPartial Exchange of electronsFormation of Molecular orbitalsIonic bondingDirectionalTransfer of one or more electron from one atom to the otherDifference of Electronegativity (capacity to attract electrons) defines the type of liaison

5 Metallic Bonding In a solid, a huge number of atoms:Many molecular orbitals together lead to the formation of bands (conduction, valence,…)Some electrons are delocalized and form a cloudIs the origin of the properties of the solid: conductivity, optic, magnetic properties,...Electrons cloudAtom

8 Surface Free Energy Creation of a surface Driving force for solidsYou need energy to create a surface!You break chemical bondsWork to create a surface define the free surface energy γThermodynamically, every system want to decrease its surface energyDriving force for solids

9 Surface Free Energy (2) Minimizing the surface free energy:1. By reducing the amount of surface area exposed2. By predominantly exposing surface planes which have a low surface free energy3. By altering the local surface atomic geometry in a way which reduces the surface free energyAggregation of the particlesCrystal ShapesRelaxation/ReconstructionAll surfaces are energetically unfavourable in that they have a positive free energy of formation. A simple rationalisation for why this must be the case comes from considering the formation of new surfaces by cleavage of a solid and recognizing that bonds have to be broken between atoms on either side of the cleavage plane in order to split the solid and create the surfaces. Breaking bonds requires work to be done on the system, so the surface free energy (surface tension) contribution to the total free energy of a system must therefore be positive.The unfavourable contribution to the total free energy may, however, be minimised in several ways :1. By reducing the amount of surface area exposed2. By predominantly exposing surface planes which have a low surface free energy3. By altering the local surface atomic geometry in a way which reduces the surface free energyy of a system must therefore be positive.The unfavourable contribution to the total free energy may, however, be minimised

11 Relaxation/Reconstruction (1)adjustments in the surface layers spacings perpendicular to the surfaceReconstructionchange in the periodicity of the surface structure and surface symmetryUnrelaxed surfaceRelaxed surface (d1-2 < dbulk )

12 More realistic case (Thin films)Solid-solid interface(a) and (b) are abrupt interfaces since there is no mixing that occursThe non-abrupt interfacesmixing (or interdiffusion)reactive (forming new chemical compounds, possibly multiple phases, the stability of which are dependent on thermodynamic parameters)

13 Number of atoms doing transitionsT-L-K ModelDescribes the structure of equilibrium surfacesAssumption: all bonds are equal in the solidT=TerraceL=LedgeK=KinkEx: move an atom from a terrace site to a kink siteDifference: the energy of two bondsNumber of atoms doing transitions

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